Although most of choosing a computer case deals with aesthetics there are a few things that buyers should keep in mind before making their purchase. In addition to visual appeal, the size, manufacture material, and number of peripheral device bays are also of great importance.

It's a good idea to keep in mind space restraints when buying, as they come in different sizes. Tower cases come in 3 sizes, mini, mid, and full. Also, the size of the case limits the number of components you can add. My full size case has room for 5 or 6 CD, DVD, etc drives in the front, whereas, a min-tower will only hold 2 drives in the front, perhaps only one. I am sure that some there are a few people out there that NEED to have space for that many extra drives, but I don't and neither does the typical user. At most you may want a floppy drive, a DVD/CD rw drive, and maybe a DVD/CD drive, a card reader, or perhaps a stereo component like the Creative Sound Blaster X-Fi Titanium Fatal1t. (the sound blaster is just a cool extra)

These very small computer cases are great for small offices and rooms. Since these towers are smaller, they require smaller motherboards.  If you already purchased or own a motherboard you may want to check it's measurements to make sure your it will fit into this type of case.

Finding a Motherboard to Fit Your Computer Case

Other topics to keep in mind when choosing a tower case are the power supply and the case material. Cases are made from aluminum, steel, or acrylic, although one day I intend to build myself a nice mahogany case. When choosing a case it is good to know that the material affects the aesthetics (how it looks), cooling, price, and weight. Steel cases are inexpensive, but heavy, aluminum light and easy to cool, and acrylic have poor heat conductivity but are generally prettier and easier to mod.

Since the power supply is so important, I recommend reading Choosing and Installing a Power Supply, and not worrying about whether or not the case you choose comes with a power supply already installed. Cases with a stock power supplies already installed typically have weak power supplies, about 250-450 watts, it is probably best to remove this and upgrade.